Confidence interval: sample mean and population standard deviation

It may be assumed that the mass of precious metal found in a randomly selected cubic metre of soil is normally distributed throughout a mine.

Sixteen samples are taken from random locations within the mine. Each sample consists of one cubic metre of soil, independently extracted and analysed. The number of grams of precious metal in each sample is then recorded.

From the sample results, a 95% confidence interval of $(52.65, 67.35)$ is calculated for the population mean using $z = 2$.